1. Field of the Invention
This invention relates to a process for forming a silicon dioxide film. Since the silicon dioxide film obtained by the process of the present invention can be formed by the use of polymer materials soluble in an organic solvent, a base step can be made flat. Since it is not oxidized by an oxygen plasma treatment employed during a fabrication process of semiconductor devices, it can be used as an insulating film in place of, or in combination with, a CVD film that has been used in the past. The silicon dioxide film obtained by the process of the present invention can be used at a greater thickness than an inorganic SOG material, and can make a base step sufficiently flat.
2. Description of the Related Art
With the improvement in the integration density of semiconductor integrated circuits in recent years, a surface step after device formation has become greater, and the thickness of wirings must be increased so as to prevent a drop in wiring capacity resulting from miniaturization of wirings. The latter means that the step after wiring becomes greater. Accordingly, a formation method of an insulating film capable of providing excellent flatness has become necessary. To satisfy this requirement, an inorganic SOG consisting of trifunctional siloxane units or an organic SOG consisting of tetrafunctional siloxane units and trifunctional siloxane units is used as a flattening material for an inter-level insulating film. However, since the inorganic SOG is likely to undergo cracking due to heat-treatment, it; is used in the form of a thin film of below 0.1 .mu.m on a wiring substrate. The organic SOG is likely to undergo oxidation cracking due to oxygen plasma treatment, and is therefore used in a sandwich structure in combination with a CVD film using an etch-back step. A process in which polysilazane is heat treated and used as a silicon oxynitride film (Japanese Unexamined Patent Publication (Kokai) No. 62-88327) and a process in which polysilazane is heat treated in a vapor atmosphere and converted to a silicon dioxide film (Japanese Patent Application No. 3-279633) have been proposed.
As described above, flattening of the base step by the use of the SOG materials has been examined in the past. However, since the inorganic SOG can be used only in a thin film of below 0.1 .mu.m, a satisfactory flattening effect cannot be obtained for the base step. Even when used in a relative thick film of not greater than 0.5 .mu.m, the organic SOG does not undergo cracking due to the heat-treatment, but cracks easily occur when the organic SOG is oxidized by the O.sub.2 plasma treatment used for the fabrication process of the semiconductor device. For this reason, a process of use which does not expose the SOG film on the through-hole side surface, that is, as shown in FIG. 1, a process which coats SOG on the CVD film and then etches back the entire surface so as not to leave the SOG film on the step, has been employed. In FIG. 1, 1 denotes a silicon substrate, 2 denotes an aluminum wiring, 3 denotes an organic SOG film, and 4 denotes a CVD film.
The silicon oxynitride film formed by using polysilazane, that has already been reported, involves the problem that emission of an amine type gas is observed at the time of re-heating (FIG. 3). According to the process which effects heat-treatment in the vapor atmosphere, corrosion of the aluminum used for the wiring material due to vapor or the amine type gas is observed. Therefore, this process is not free from the drawback that a film having high water resistance formed by an expensive apparatus such as an ECR plasma CVD apparatus must be used.